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CHAPTER 7

Water Storage

 

  IT has already been stated that the greatest available water storagecapacity exists in the soil itself. The association of Keyline cultivation and thiswater storage capacity has already been explained.

  If all rain which falls on crop and pasture land could be absorbedinto the soil, there would still be areas remaining that do shed most of the rainthat falls on them. Farm roads and yards, the homestead and other farm buildingsand sheds, and often main roads, shed considerable quantities of rain. Conservationof this water for farm use is of the utmost importance.

  Whether a farmer realises it or not, he is dealing with forces thatneed the full use of engineering planning. A sudden storm may send 100,000 tons or500,000 tons of water on to a 1,000-acre area in an hour or two. This huge weightof water can be controlled and conserved by the farmer to the great benefit of theland and himself, or it can run largely to waste, leaving a trail of destructionin its path.

  Levels are important factors in any water control and conservationproject. These need to be used to advantage by the farmer. Contours and other levelconsiderations are basic land engineering factors. The farmer must know how and whento use them.

  The application of Keyline methods requires very little levellingwork, but those levels that it does require are of great importance.

  On undulating country, dams can usually be located which will enablethe farmer to enlist the forces of gravity to provide him with water under pressure.This will give him a better farm, easier work and higher yields. Other things beingequal, the value to a farmer of conserved water is in direct proportion to the heightof the storage. The dams of potentially greatest value are those in his high country.

New pasture being irrigated by gravity pressure on "Nevallan". Guidelinetimber strip forms a permanent market for Keyline cultivation. The truck in the centrebackground is travelling along our Keyline road.

 

  The Keylines, by crossing the valleys at their first main point ofslope flattening, will invariably position the highest suitable valley storage areafor water.

  In any plan of general land development, the control of water is oneof the first considerations. At the same time, it is to be kept in mind that KeylineAbsorption-fertility is going to reduce run-off water very considerably. It may evencompletely stop run-off water from farm and pasture paddocks, except in the rare,but under present conditions, very dangerous period of general heavy flood rains.With the absorption of what previously would have been heavy run-off, considerationhas to be given to conserving water from every available source.

  With the Keyline positioning the highest suitable dam sites, it becomesimportant to locate potential water-shedding areas above the Keyline.

  The Keylines have been illustrated as contour lines in the discussionon cultivation for the sake of simplicity.

  For purposes in connection with the conservation of water in the Keylinedam, the Keyline itself is a gently falling line to form a drain or water race tocarry water to the dam. The use of the Keyline, which is now a drain, is still fullyeffective as a guide for Keyline cultivation.

  It is usually convenient and good practice from most other viewpoints,to locate the homestead and all farm buildings and the yards and their attendantroads in the higher country. From the point of view of full Keyline development,it becomes a part of planning to do so, in order to secure abundant run-off waterto fill the Keyline dams from these sources.

  Wherever it is possible and practical, dams are constructed on theKeyline in the valleys, and the Keyline itself is pegged and constructed as a gentlyfalling drain to carry water to the Keyline Dams.

  Keyline dams are constructed with a pipeline through the wall or throughthe floor to one side of the centre line of the valley, so that the full gravitypressure of the conserved water is available for spray irrigation and other farmpurposes.

  Where areas of land exist that are 50 feet or more vertically lowerthan the Keyline, the water from the Keyline dam will supply effective pressure forirrigation without pumping. This "line of effective water pressure" suitablyforms the top boundary for the irrigation paddocks. A 4-inch pipe through the wall,controlled by a 4-inch gate valve, in these circumstances will control gravity pressurewhich, often from a single dam, will effectively operate a comprehensive spray irrigationand stock-watering system.

Spray irrigation without pumping. The Keyline dam which provides the wateris situated in the hills below the small cloud in the centre of the picture. Thebare area on the right is located on the site of an old disused road. All land belowthe timber line can be spray-irrigated wit4hout pumping costs.

 

  With the use of a 4-inch pipeline, the vertical drop from the waterlevel to a nearby irrigation area multiplied by 0.4 will give the approximate poundspressure available in the spray line. A vertical fall of 50 feet multiplied by 0.4gives a twenty pounds per square inch pressure, which is suitable for operating mosttypes of spray lines. As the spray line is moved downhill a little on each "move",there is, of course, an increase in available pressure.

  Referring to Map 4, which exhibits the same land area as Map 3, theKeyline crossings of the valleys are to be considered as possible dam sites. Thesites marked in four of the valleys could be considered good dam sites. The siteof the Keyline crossing of the fifth valley is not as suitable as the others.

  The most valuable water storage site for a Keyline dam is locatedin the first valley, as this site has the greatest area of land below It which Issuitable for irrigation by gravity sprays. This fact indicates a rule or generalformula for determining the direction of flow of the Keyline when it is formed bya drain. If the creek or drainage line below a series of valleys--as in Map 4--hasa general fall greater than five feet per thousand feet--the fall recommended forthe Keyline drain--the direction of the Keyline fall follows that of the creek. Whenthe creek has a flatter fall than required by the Keyline drain, the drain fallsin the direction opposite that of the creek. This is illustrated by the shaded areaon the map.

  The construction of a Keyline dam will often cost considerably lessthan a pump and engine installed for spray irrigation. The Keyline dam, its pipeand valve outlet, will operate the same sprays with no pumping cost.

  This low cost water is used in the general programme of progressivesoil development, and higher yields will be incidental and automatic to the KeylineAbsorption-fertility programme.

  The following construction comments should be considered.

  Most undulating country is suitable for dam construction if correctpreparation and compaction of the material in the wall is secured. Fine clay, whichis usually considered the best material for dam bank construction, has its own particularproblem. This material in the wall of the dam will tend to "jell-up" belowthe waterline to such an extent that the weight of the wall above this wet unstablematerial may squeeze the material outwards from the wall, thus causing a centralsubsidence of the wall which extends down below the water line. This would resultin the water overflowing at this point and would completely destroy the bank.

  In shale country the mixture of shale and clay will give the bestpossible material for bank construction.

  Before laying in a dam bank, the foundation area of the bank mustbe treated first according to the type of country. In shale country it is necessaryto remove only the darker topsoil material to one side-this can be used later tocover the bank to obtain a quick growth of grass. This cleared area is then rippedbefore the wall filling material is placed on it. The material for the wall shouldbe placed on in layers of from 6 to 12 inches thick, so that suitable compactionof the soil takes place during construction. Bulldozers will give sufficient compactionusually without the need of further special compacting implements.

  The back of the wall of the dam, that is the side away from the water,should not be specially compacted. If water seeps through the compacted front ofthe wall into the centre, it must be allowed to get out through the back of the wall,otherwise it may build up hydrostatic pressure inside the wall. This could destroythe wall by forcing or breaking the material from the back of it.

  Clean water seeping through a dam wall is usually quite safe, buta seepage that is discoloured by the wall material should be considered a dangerto the wall itself. Raking or harrowing of the side of the wall in the water of thedam is usually the best means of sealing this type of seepage.

  In the construction of this type of dam by bulldozers, the excavationof the sides of the dam, if the land will stand firm, should be made on as steepa slope as the implement will dig. The water-side of the wall, as formed by the actionof the bulldozer pushing the material upwards, should be flatter than the excavatedsides. Usually the limitations of the implement to push material up the slope ofthe wall will form a wall of suitable slope.

  The laying of a pipeline through the wall of the dam, or through theearth below the wall of the dam, requires some special attention.

  The danger to be avoided here lies in the fact that water will tendto flow along the outside of the smooth pipe, creating an ever-widening and largerhole, which may eventually let all the water go and so destroy the wall.

  The following method of laying these pipes has been found completelysatisfactory.

  After the wall site has been prepared by clearing away the topsoilmaterial and the subsoil ripping, a trench to receive the pipeline is dug

  across the wall area a little to one side of the centre line of thevalley fall. This trench is to be at least three times the diameter of the pipelinein width and depth.

  A 12 x 12-inch trench is required for a 4-inch pipe. The pipelineis laid in this trench with three or four large loose flanges 12 to 16 inches diameter.These are placed around the pipe from the inside of the wall to about its centreline.

  At each flange along the pipeline trench, two or three bucketfullsof wall material mixed with about 20 per cent. of some lightweight material is placedaround the pipe. The trench is then filled in with adequate tamping of the materialup to the surface level of the trench. It is important that this material shouldhave the same moisture content as the wall material.

  The special mixture at each flange of the pipes will tend to sealthe leak .if water does commence to flow along the outside of the pipeline. Someof this lightweight material will move to the small openings and will automaticallyre-seal them.

  In granite country it may be necessary to excavate a considerableportion below the wall site down to the depth of the firmer decomposed rock to preventcomplete loss of water through the material below the wall..

  If this work is done properly and the bank consolidated in layersof six to nine inches deep, dams in this country will hold water effectively. Withoutthis work these dams will often not hold any water.

  The High Contour dam is the highest dam of the Keyline plan. It islocated in the areas above the Keylines.

  Gently sloping country usually exists above the steeper slopes whichlie above the Keyline. The valley heads will actually start at the low edge of thisflatter country where the steep slope country commences. The High Contour dam isconstructed here. The area selected for the dam site can be the side of a hill orridge. A slope as steep as 1 in 10 is suitable.

  The race or drain to transport water to fill this dam is located abovethe valley heads. It also serves to protect further these valleys by preventing anyflow into them. The drain requires a fall of approximately 5 feet per 1000 feet.The site of the drain and dam must be studied and planned together.

  A sketch and cross section of a High Contour dam built on the steepslope mentioned is illustrated below. Each cubic yard of earth moved conserves twocubic yards of water. This ratio is not as favourable as that in the constructionof Keyline and other valley dams which may be round the ratio of six of water toone of excavated material. However, the value of the conserved water in this HighContour dam more than warrants its construction where the topography is suitable.

  The High Contour dam may be constructed anywhere along a ridge wherea suitable slope exists and where run-off water can be brought to the dam by a drainfrom one or both directions.

  Because of these circumstances, the dam is usually long and narrowand always along the contour.

  A bulldozer is used for construction and the earth is moved from thetopside straight across the dam to form the wall. In this way the haul is lessenedand the cost of earth moving is in direct proportion to the distance the earth ismoved, so this distance is kept to the minimum.

  The drain to fill the dam is located and pegged when the dam is markedout. The construction of the dam is completed before the drain is built. There isthen no danger whatever of losing from heavy rains any part of the dam during itsconstruction. The back wall of the dam is constructed first. Then the 4-inch pipeoutlets are laid at one, or both, ends. After this, the end walls are closed andthe drain made.

  A spillway is not constructed, because surplus water is allowed tooverflow from the drain at some distance from the dam when it is full.

  It is only necessary to see that the overflow does not occur at thesame place more than once during the first year or two, so that no water wash isstarted. Once the drain is grassed, blocks can be made at any suitable place in thedrain to overflow the water there.

  Water transporting drains can become less effective, or sometimescompletely ineffective, by becoming overgrown with vegetation. The best means ofcontrolling this growth is by seeding the drain to good grass species and manuringthe drain heavier than the adjacent pasture. This encourages the stock to graze thedrain area more closely than the rest of the paddock. It is also advantageous tomow regularly the long excavated slope of the drain so that the water transportingcapacity of the drain is unimpaired.

  If a road is to traverse the area of the drain it can be placed parallelto and above the drain. The water run-off from the road is caught by the drain andconserved.

  The Keyline dam, constructed on the Keyline, and the High Contourdam, above the Keyline, are the two highest dams used in Keyline planning. For thisreason they are the most important dams of all water-conservation schemes.

  The water conserved in these dams is available under pressure forinstant use. It is the lowest cost irrigation of all conserved water and is, therefore,used when the first dry spell makes its use profitable and advisable. No dam shouldever be completely emptied except for reconstruction or enlargement. A few feet ofwater is always left in these dams, and this will go a long way toward protectinga bank from dangerous dry cracking.

  There are many farms that do not have their own Keylines. The developmentof these farms is mentioned in a later chapter. The con servation of water belowthe Keyline and on these properties of lesser slopes is discussed here.

  The first of these dams is called the Guideline dam, and is, likethe Keyline dam, a valley dam. The wall material is excavated from the area of thevalley which will be below water level when the dam is filled. All earlier commentsabout the Keyline dam, including the pipe outlet, are common to this dam. Its particularlocation is apparent from the chapter "Flatter Lands".

  The next dam in Keyline Planning has its counterpart in the ordinaryvalley dam. These are to be seen on farms and grazing properties all over the countryside.The main consideration in locating the usual farm valley dam has been to conservethe greatest amount of water for the earth moved.

  With the absorption into the soil and the conservation in Keyline,High Contour and Guideline dams, of practically all the rainfall, a large capacitylower dam has to be located where it can be filled despite these other storages.By locating it in a lower valley, such as the site indicated on Map 4, it is in afavourable position to receive the combined seepages from all the higher country.Apart from seepages, this dam will receive water from very heavy storms and in theperiods of general heavy flood rains when most water conservation storages may overflow.

  These dams can be made large to act as a buffer or safety againstprolonged drought. They should be as deep as practical, so that evaporation lossesare reduced. Losses by evaporation are in proportion to the surface area of the water.A dam six feet deep could lose all its water in a hot dry year, while a deeper damwould lose only the same depth and have water storage when the other is empty.

  The construction of dams by blocking a stream or creek is usuallycontrolled by the Government Water Conservation and Irrigation Authority. Plans forthese usually need the approval of this authority, which will also often assist withadvice on the preparation of the construction plans.

  Apart from other constructional details, the provision of adequateand safe spillways for overflow is of maximum importance in these stream dams.

  Contour dams, of which the High Contour dam is the one placed in thehighest location, can be constructed in almost any type of country to provide lowcost large capacity water storage. They are not located in valleys and, as with theHigh Contour dam, require drains to provide the water.

  On the land below the Keylines they can be filled from a flowing streamor one that flows intermittently.

  The location of a Contour dam is decided by first, the means to fillthe dam, and second, a suitable area for the use of the conserved water. The watermay be used for spray irrigation and other purposes. The main excavation and bankof the contour dam is always along the contour. The total cross sectional area ofthe excavation and bank are approximately the same whether the dam is very largeor of medium size.

  In the construction of the Contour dam a bulldozer is used and earthis moved straight down the slope at a right angle to the contour. The distance ofthe "haul" is kept to 100 feet approximately, to provide for the most efficientbulldozer operation.

  A similar construction to that of the High Contour dam is followed.In flatter country the end walls--which are the same length as the width in the HighContour dam--become longer. In the High Contour dam all water is conserved by holdingit in the excavated area by the wall. The Contour dam, on the other hand, holds muchof its capacity over unexcavated land.

  In flatter country, where the contour dam then assumes the shape ofa "broken ring", the end walls are turned in toward each other. The waterrace feeds the water into the dam between the converging end walls.

  On still flatter sites it assumes the shape of a "complete ring"and the major storage capacity in larger dams is then over the unexcavated centralarea.

  A pipe outlet is placed through the end wall of the Contour dam atthe lowest ground level, and water conserved above this height can be released bygravity.

  Gravity pressure is used for irrigation if the conserved water ishigh enough.

  The outlet pipe through the wall of these dams can lead directly toa centrifugal pump outside the wall. This maintains the pump under a positive waterhead, so that instantaneous water pumping is available without pump priming.

  A Complete Ring dam should be constructed on a flat area of land belowa ridge to which water can be brought by the drain. In deciding the location of theComplete Ring dam consideration is first given to the filling of the dam by flowfrom a watercourse. It may be practical to lead water from a watercourse along awater race to a rise close to and above the site, and from this point flow the waterover the wall through fluming.

  The filling operation is controlled by a low weir wall constructedacross the supply stream bed. A suitable notch outlet is provided to control thewater. This wall, constructed of logs, grouted stone or cement, need be only 2 feetto 3 feet high.

  The fluming for the Complete Ring dam may be made of a variety ofmaterials, but its shape is always that of a long trough. Wood or iron fluming ismost suitable and the fluming is supported by a trellis of bush timber.

  These dams, ranging from the High Contour to the Complete Ring damare suitable for easy construction and very profitable use in a wide variety of farmingland. Small bulldozers may be used. All the land that can be spray irrigated fromsuch dams will develop rapidly in fertility, productiveness and value. Keyline progressivesoil development, greatly stimulated by the correct use of spray irrigation, willbring this land very close to the value class of fertile irrigable river flats.

  The overall costs of spray irrigation will be less than those pertainingto river flats and the pumping of water from the river. River water will have tobe "lifted", whereas the water of these dams is at least "assisted"by gravity.

 


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